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Interactions of Cell-Penetrating Peptides with Membranes 171
FIGURE 8.1 Variations of the surface pressure with the mean molecular area of mixed
DOPC- 1 monolayers at various peptide molar fractions (x P). (Adapted from Van Mau, N. et al.,
J. Membr. Biol., 167, 241, 1999. With permission.)
of the membrane uptake of this peptide was undertaken using mainly monolayers
AFM and FTIR. 57,58
8.2.2.8.1 Monolayer Study
Because of its solubility properties, the peptide could be dissolved in the same
volatile organic solvent mixture as the phospholipids (DOPC, DOPG, DPPC, and
DPPG) and, hence, the peptide–lipid mixtures could be spread at the air–water
interface. The various compression isotherms at various peptide molar fractions were
recorded and are shown in Figures 8.1 and 8.2.
Analysis of the isotherm was made by plotting the variation of the mean molecular
area as a function of the molar fraction at a given surface pressure. In all cases
the variations of the mean molecular area showed positive deviations from additivity
(Figure 8.3 and insets of Figure 8.2). 109,110 Such behavior can be considered indicative
of strong peptide–lipid interactions with expansion of the mean molecular area.
However, the deviation from linearity is larger in DOPC and DOPG than in DPPC
and DPPG, suggesting that the nature of the peptide–lipid interactions could strongly
depend on the nature of the fatty acid. Further investigations, based now on FTIR,
showed clearly that the peptide underwent a conformational transition, going from
an α-helical form at low peptide molar fraction to a β-type structure at higher molar
fractions. This conformational transition was detected by examination of the position
of the Amide I band, the major component of which moves from 1655 cm –1 (wavenumber
characteristic of an α-helix) to 1625 cm –1 (wavenumber characteristic of a
sheet structure) 111,112 upon increasing the peptide molar fraction (Figure 8.4). This
holds true, at least, when the peptide interacts with DOPG, but probably also for
DOPC. However, a transition occurs at very low peptide molar fractions, probably